Acoustic waves, e.g., ultrasonic waves, find use in a variety of applications. In many such applications, it is desired to manipulate the acoustic waves in order to focus, disperse, steer, waveguide, or otherwise alter them. There are several different techniques used to fabricate materials with gradients in material properties, such as ion exchange, chemical vapor deposition (CVD), sol-gel, etc.
Chemical vapor deposition techniques can be used to fabricate a gradient in material properties. CVD requires that the reactants be present in the gaseous phase and that the reactants are able to react with each other. This places a restriction on the choice of chemicals that can be used in the fabrication of the final gradient profile, thereby reducing the ability to generate specific property gradients as a function of thickness. Also, this technique is typically used when impurity levels in the material need to be restricted to part per billion (ppb) levels. Due to this fact, this technique is highly capital intensive and time-consuming. Although, the use of CVD can be extended to acoustoelectronics, acoustooptics, and other ultrasonic engineering applications, its use is primarily constrained to fiber optics, photonics, and the semiconductor industry.
Sol-gel processes can also be used to fabricate a gradient in material properties. Unlike CVD, this technique involves the reaction of constituents in the liquid phase. The formation of the gel is restricted by the kinetics of the chemical reaction, which limits the available reactants for production of the gradient profile. Furthermore, failure due to drying shrinkage places a limitation on the dimensions of the final product and is an inherent disadvantage to this technology.
Surface modification for the generation of property gradients in crystals and glasses can be achieved by diffusion of ionic species into or out of the substrate material. The case where the inward diffusing species (ions) replaces the outward moving species, is referred to as ion exchange. A recent publication by A. Abramovich entitled "Acoustic Properties of Gradient Glasses", published in the proceedings of the 18.sup.th International Congress on Glass, discloses the use of optical glasses having a gradient in the optical index of refraction and formed by ion-exchange for acoustoelectronics, acoustooptics, and other ultrasonic applications. In glasses, the diffusion of ions occurs through the interstitial volume of the glass structure, whereas in crystals, the diffusion occurs through the interstitial lattice. The diffusion of ions into glass or crystalline materials usually follows Fick's laws of diffusion. According to Fick's laws, the rate of ion diffusion decreases with time and increases with temperature. In glasses, the maximum diffusion temperature is determined by the glass transformation temperature (T.sub.g) of the substrate glass. Thus, obtaining large diffusion depths at a temperature below T.sub.g becomes difficult for reasonable amounts of time. In crystals, the diffusion mechanisms are more complex and their description is outside the scope of this invention. However, it will suffice to say that the time and temperature relationships in regard to the depth of ion penetration are similar to that of glasses.
Surface modification of the material substrates using ion exchange or solid state diffusion imposes several limitations with regards to obtaining a particular property gradient. Firstly, the maximum total change in a particular property is dependent upon the diffusing species and does not result in any major structural changes in the material. Therefore, the total change is typically very small. The propagating wave would not see a large difference in acoustic velocity (transverse or longitudinal); therefore, there exists only a limited ability to manipulate the acoustic waves. Secondly, this technique offers the ability to create a variety of functional distributions of a material property, but does not offer the flexibility to tailor a specific material property profile. Thirdly, the total distance across which the material property gradient exists is limited due to the nature of ion exchange phenomena and solid state diffusion. This does not allow for the fabrication of material property gradients across a large thickness.
Therefore, there is a need to provide a tailored element for manipulating acoustic waves, with a gradient throughout its thickness and the flexibility to obtain this gradient across a large thickness. By "large thickness" herein is meant a thickness on the order of about 6 to 30 mm, which is to be compared to the gradient thickness achieved by ion exchange, which is at most only a few mm.